[Solved]-Highcharts border curve inside-Highcharts

That is not possible by using defualt Highcharts API, but there is a possibility to add an extension by wrapping prototype functions. You can wrap drawPoints method to change column shape type from rect to path with curved top border.

(function(H) {
    H.wrap(H.seriesTypes.column.prototype, 'drawPoints', function(proceed) {
        this.points.forEach(function(point) {
            var sArgs = point.shapeArgs;
            point.shapeType = 'path';
            point.shapeArgs = {
                'd': [
                    'M', sArgs.x, sArgs.y,
                    'Q', sArgs.x + sArgs.width / 2,
                    sArgs.y + sArgs.width / 2,
                    sArgs.x + sArgs.width, sArgs.y,
                    'L', sArgs.x + sArgs.width, sArgs.y + sArgs.height,
                    sArgs.x, sArgs.y + sArgs.height, 'z'
                ]
            }
        });

        proceed.apply(this, Array.prototype.slice.call(arguments, 1));
    });
}(Highcharts));

Live demo: http://jsfiddle.net/BlackLabel/as6y1uhn/

API Reference: https://api.highcharts.com/class-reference/Highcharts#.wrap

Docs: https://www.highcharts.com/docs/extending-highcharts